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Fighting herbicide-resistance

Herbicide resistant weeds aren’t a novelty anymore — now they’re just the norm

While herbicide resistance isn’t a new problem, it is a growing one. Today 80 countries report herbicide-resistant weeds, says a researcher. And Canada is near the top of that list.

Dr. Ian Heap heads the international survey on herbicide-resistant weeds, and reported those findings at the Herbicide Resistance Summit in Saskatoon this March.

“North America as a whole has more herbicide-resistant weeds than any other continent. And Canada is actually quite high up on that list,” he said during an interview.

In fact, Canada ranks third globally when it comes to the number of herbicide-resistant weeds.

Why does Canada rank so high on that list?

“It’s really because you have a massive land area and you’ve been using herbicides for so long that you’ve been selecting herbicide-resistant weeds,” Heap said.

Closer to home, researchers with Agriculture and Agri-Food Canada are surveying farmers’ fields to see how much resistance has eroded herbicide efficacy in Western Canada.

Dr. Hugh Beckie presented findings from the 2014-15 Saskatchewan weeds surveys, which were led by Dr. Julia Leeson.

Researchers do the surveys every five or 10 years, he said. Manitoba is next on the list, and they plan to scout Alberta fields in 2017, Beckie said. Researchers survey fields in July, after weed management is done, so they can look at weeds farmers are not controlling, he added.

Researchers did the baseline surveys in the early 2000s, and found herbicide-resistant weeds on about 20 per cent of fields, Beckie said. They haven’t finished screening for herbicide resistance from the latest survey, but Beckie expects “well over 50 per cent of cultivated land in Saskatchewan will have a herbicide-resistant weed.”

“Certainly the trend is continuing upward, which is what our model predictions indicate. We don’t have any new herbicide chemistries in the last 30 years and so growers have had to use the same tools to manage their populations, whether they’re resistant or not. So inevitably, if we don’t have any new modes of action, we’re finding that they continue to increase.”

Top 10 Sask weeds

The latest weed survey revealed the 10 most abundant weeds in Saskatchewan.

Beckie noted that narrow-leaved hawk’s beard jumped from No. 20 in 2003 to its present spot on the top-10 list.

“I know on our farm near Davidson it’s become a real problem weed that’s fairly recent — in the last 10 years,” said Beckie.


The 10 most abundant weeds in Saskatchewan include:

  1. Wild oats
  2. Green foxtail
  3. Wild buckwheat
  4. Volunteer canola
  5. Canada thistle
  6. Spiny annual sow thistle
  7. Cleavers
  8. Lamb’s quarters
  9. Narrow-leaved hawk’s beard
  10. Dandelion

Beckie considers cleavers “the most problematic broadleaf weed with resistance in Western Canada. Back in 1970 it ranked 50th. Now it’s No. 7. It’s the weed that’s increasing the fastest in the last 50 years.”

Spiny annual sow thistle leaped from No. 34 in 2003 to No. 6. Beckie said he’s never seen a jump like that before. He noted that there are ALS-resistant populations in Alberta.

“And we’re finding in our surveys in Saskatchewan that there’s a lot of ALS resistance. This may be part of the problem in terms of control,” said Beckie. Spiny annual sow thistle seed is also very light, and can travel high up in the atmosphere, he said. That makes seed dispersal a problem.

Volunteer canola also moved up, from No. 16 in 2003. Beckie said that increase probably reflected more intense canola rotations.

As for the top three weeds (wild oats, green foxtail, and wild buckwheat), Beckie said they haven’t moved in the last 15 years, despite availability of ACCase and ALS inhibitors.

Farmers and their agronomists also submit suspect weeds for herbicide-resistance testing. Between those samples and the weed surveys, it’s safe to assume Group 1 resistance in wild oats is everywhere, Beckie said.

“I’m actually surprised growers still submit samples for testing because I would assume as a grower we have resistance,” he said. But, he added, farmers are trying to figure out which Group 1 still works on that field.

Beckie estimates that at least 20 per cent of farmland in the Prairies now has multiple-resistant populations of wild oats.

Beckie appreciates growers sending in samples. Field surveys miss many of the early cases, but suspected herbicide-resistant samples gives researchers a heads up, he said.

What about kochia?

Kochia seeds aren’t viable pre-harvest, so the weed has to be surveyed after harvest. But kochia is certainly on researchers’ radar. Based on survey data, Beckie estimates there are several hundred cases of herbicide-resistant kochia.

Beckie summarized research looking at how kochia weeds are dispersed. Researchers at Scott and Lethbridge fitted kochia tumbleweeds with small GPS collars, then set them free to go where the wind would take them. They noted how many seeds the tumbleweeds had at the beginning and end of the experiment.

At Lethbridge, it was quite common for tumbleweeds to travel a kilometre, Beckie said, and they would have tumbled on if it wasn’t for a fence. At that distance, they’d dropped 80 to 90 per cent of their seeds. Beckie added that the faster the tumbleweeds rolled, the more seeds dropped as well.

At both the Scott and Lethbridge sites, researchers found kochia could shed up to 100,000 seeds per plant. And most, if not all, of those seeds were viable.

Kochia is an outcrossing weed so resistance can also be spread through pollen, Beckie said.

Beckie noted they avoided using glyphosate-resistant kochia in the experiment.

Top 10 weed management practices

Beckie said it’s challenging to get farmers to implement herbicide-resistance BMPs because growers are diverse, and one size doesn’t fit all. But through the surveys, Beckie has found that growers who use BMPs tend to have less herbicide resistance.

Beckie presented his top-10 BMPs, in the spirit of David Letterman’s top-10 lists.

10. Sound record-keeping. Include weed populations in each field, not just herbicide use.
9. Strategic tillage. Researchers found that growers who included some tillage, even infrequently, had less resistance. Beckie said there is a place for some tillage, especially with weeds such as kochia. Vertical tillage and rotary hoes are two options.
8. Site-specific weed management. Manage resistant patches early, before they spread through the field, Beckie said.
7. Weed sanitation of equipment.
6. Don’t use wheat-selective in-crop herbicides every year on wild oats. Wild oat uses the same enzyme system to break down herbicide as wheat. Using wheat-selective herbicides repeatedly will select for resistant wild oats.
5. Rotate herbicide groups.
4. Use tank mixes.
3. Pre- and post-herbicide scouting.
2. Select competitive crops and implement practices that promote competitiveness.
1. Crop rotation. Overall, Western Canada does have relatively diverse crop rotations, despite canola being grown frequently. “And I think that has kept the lid on resistance in Western Canada, along with some of the other practices.”

Beckie said he can’t say enough about the importance of monitoring herbicide resistance.

“We really have to keep our eyes open.”

Click these links for video interviews with Hugh Beckie and Ian Heap.

About the author

Field Editor

Lisa Guenther is field editor for Grainews based at Livelong, Sask. You can follow her on Twitter @LtoG.

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